Conventional cardiac pacemakers and defibrillators generally consist of a generator for electrical stimulation and an elongated flexible pacemaker lead that is connected proximally to a header structure on the generator which is implanted distally from the heart for cardiac pacing and defibrillation. The cardiac lead is generally configured with tubular electrically insulated sleeve structures that are inserted into the body through an incision overlying veins leading to the heart chambers where the distal end of the lead is lodged. In such cases, the distal end of the lead is connected to a tubular tip electrode, having an increased diameter forming an annular shoulder against which the distal end of the sleeve abuts.
Biocompatible silicone based adhesives are generally used to connect the distal end of the lead sleeve and the tip electrode. Among the limitations of adhesives is that the manufacture of the assembled lead requires sufficient time for the adhesive to cure, and the adhesive's bond strength may decrease in time and permit separation from the tip electrode from the sleeve. Fixing the distal end of the lead to cardiac tissue is accomplished generally by conventional anchoring systems. One such active fixation mechanism involves a screw-in electrode and further there has been used a passive fixation mechanism consisting of one or more radial tines that engage the inner lining of the heart or blood vessel.
Such conventional devices are typically employed and include a single chamber device as well as a dual chamber device. The single chamber device is capable of sensing and pacing in one chamber, either in the atrium or in the ventricle. Dual chamber devices have the capability of sensing and pacing in both chambers. Modes of pacing include VDD, DVI, VVI, and DDD, where the first letter of the mode indicates the chamber being paced, with the second letter indicating the chamber being sensed, and the third letter indicating inhibited or triggered responses. A fourth letter “R” may denote rate responsive pacing to match a patient's activities. In addition to pacing the right/atrium and ventricle pacing, the left ventricle by way of the cardiac veins or biventricular pacing provides a physiologic and synchronous cardiac contraction which would improve cardiac function.
There are basically two types of leads which are uni-polar and bipolar leads. The uni-polar lead has a single conductor coil with typically a cathode, or negative pole, at the distal tip and an anode, or positive pole, defined by the housing of the stimulator. Electric current returns to the anode via body tissue as a current path. In opposition, a bipolar lead has two conductor coils, the distal tip forming the cathode and an annular or ring electrode located a few millimeters proximal to the distal tip. High voltage defibrillation is delivered by the one or two shocking coils which are inserted intravenously.
Pacemaker leads which have been used are generally suited for placement in the ventricle and atrium. In order to provide permanent pacing and to avoid pacemaker lead dislodgement, various methods have been used for anchoring the leads to the endocardium which is the inner lining of the heart chambers. Conventional right ventricular apical pacing alters the normal synchronization of different heart chambers, and may adversely influence ventricular function, leading to heart failure, and increased mortality.
Biventricular pacing or resynchronization requires the placement of electrodes within the venous system of the heart. However, other than lodging the tip of the lead into the distal coronary vein, there has been found no safe anchoring mechanism to maintain the lead from dislodging. Additionally, the optimum lodging site may not be the ideal pacing location for effective myocardial stimulation. Screw-in anchors may be applied to the myocardium, but cannot be utilized in vascular structures due to the risk of endothelial damage and hemorrhage.
Conventional pacemaker right ventricular leads have the disadvantage that they must cross the tricuspid valve. Such leads may cause unwanted tricuspid regurgitation by interfering with tricuspid valve closing in heart contraction which may interfere with the right ventricular function.